Physical Factors: Rock, Soil, Landform, Water, and Wind
With this chapter we begin a more detailed exploration of the origin and maintenance of grasslands in the South. As reviewed in chapter 2, prolonged periods of drier climate or increased seasonality can convert forest to grassland. This process has occurr
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Physical Factors: Rock, Soil, Landform, Water, and Wind
The most satisfactory system of geographical classification of the vegetation of temperate Eastern North America is one based on geology. Roland Harper (1906a)
With this chapter we begin a more detailed exploration of the origin and maintenance of grasslands in the South.As reviewed in chapter 2, prolonged periods of drier climate or increased seasonality can convert forest to grassland.This process has occurred periodically in the past. Positive feedback loops involving megaherbivores and fire further contribute to grassland formation and can maintain grassland after climatic conditions favorable for its development have ended.The long sequence of grassland-adapted vertebrates in the fossil record, patterns of disjunction in plant and animal taxa between the semiarid West and the Southeast, and positive responses to fire in the flora and fauna provide evidence of this history. Other factors that maintain grassland in a relatively rainy region are physical. Although southern grasslands occur on a variety of bedrock and soil types, certain physical conditions can allow grassland to persist with few other conducive influences, for instance sometimes with little or no fire. Remnant grasslands often occur on soils that are shallow, alkaline, toxic, infertile, coarse-textured (e.g., deep sands, through which water drains quickly), or fine-textured (e.g., “shrink-swell” clay soils). Soils with any of these properties tend to favor grasses and associated plants in competition with trees. As noted earlier, grasslands associated with particular soils and associated physical conditions are called “edaphic grasslands.” One notable attribute of edaphic communities is that they may 117 R.F. Noss, Forgotten Grasslands of the South: Natural History and Conservation, DOI 10.5822/978-1-61091-225-9_4, © 2013 Island Press
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Forgotten Grasslands of the South
persist relatively unaltered despite changes in regional climate. Because edaphic communities are often isolated and tend to accumulate endemic taxa, they often represent long-term hotspots of biodiversity and centers of creative evolutionary processes. Many southern grasslands are edaphic to some degree, in that properties of their geologic substrates or soils distinguish them from surrounding vegetation. Landform often plays a role, too, with steep, erosive slopes and relatively hot and dry south- and southwest-facing slopes less favorable to forest. Also important to grassland development is hydrology. Scouring by river waters or inundation or saturation for prolonged periods can favor grassland over woody vegetation. High winds from hurricanes, tornados, or other stormy conditions break open tree canopies and promote grassy understories; with fire or other positive feedback, these grassy openings may convert to more extensive grasslands. Acting alone or in combination, these various physical factors can prevent closed-canopy forests from developing within a regional climatic regime that would otherwise favor them. Some
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